Method of manufacturing a hollow steering shaft

Abstract: A hollow steering shaft (6a) having a good quality can be manufactured at a low cost by passing a blank tube (16) having a circular cross section through a preliminary shaping die (25) and then through a finishing shaping die (26). The blank tube (16) which has passed through the preliminary shaping die (25) is formed with two kinds of convex curve surface portions (39,40) having different radii of curvature and arranged continuously, circumferentially and alternately. When the blank tube (16) passes through the finishing shaping die (26), the larger convex curve surface portions (40) are deformed into flat portions (41). As a result, a steering shaft (6a) having an elliptical cross section is manufactured.

Description: The present invention relates to a method of manufacturing, and an apparatus for manufacturing, a hollow shaft. A conventional steering apparatus of a vehicle has a structure as shown in Fig. 10, for example. A steering shaft 1 is supported only rotatably in a steering column 2 supported on a vehicle body. A steering wheel (not shown) is fixed to the upper end of the steering shaft 1. The movement of the steering wheel is transmitted to steering gears (not shown) through a universal joint 3 and a transmission shaft 4. The steering apparatus comprising the steering shaft 1 and the steering column 2 of the conventional type has a so-called collapsible structure which shrinks in its longitudinal direction when it receives a shock so that it protects the driver at the time of collision. As shown in Fig. 10, the steering shaft 1, for example, comprises a hollow cylindrical lower shaft 5 and a solid upper shaft 6 connected thereto. Each of the fitting portions of both shafts 5 and 6 has an elliptical cross section so as to prevent relative rotation therebetween. Synthetic resin members 9 are filled and solidified in annular grooves 7 formed in the outer peripheral surface of the lower portion of the upper shaft 6 and through holes 8 formed in the upper portion of the lower shaft 5. The synthetic resin members 9 prevent axial displacement between the shafts 5 and 6 in the normal operation of a vehicle. Upon collision, however, they are broken and allow the displacement between the shafts 5 and 6 to shorten the length of the steering shaft 1. Recently, there have been used more or more hollow upper shafts 6a of steering shafts 1, as shown in Figs. 11 and 12, in order to make the steering shafts light. Each hollow upper shaft 6a is manufactured by drawing a cylindrical blank tube having a circular cross section. On the upper end portion of the upper shaft 6a are formed a spline portion 10 and a male screw 11 which engages a nut for holding a steering wheel mounted on the spline portion 10. The lower half of the upper shaft 6a forms a substantially elliptical fitting portion 14 comprising a pair of arcuate portions 12 and a pair of flat portions 13 arranged circumferentially and alternately. The fitting portion 14 is inserted in a fitting portion 15 formed on the upper half of the lower shaft 5 (Fig. 10) so that only axial movement between the fitting portions 14 and 15 is allowed. The annular grooves 7 are formed in the outer peripheral surface of the fitting portion 14. Conventionally, the fitting portion 14 has been formed on the lower half of the upper shaft 6a as shown in Figs. 13A to 13E and disclosed, for example, in Japanese Utility Model N°. 59-5443 (1984). First, a cylindrical blank tube 16 to be formed into an upper shaft 6a is disposed so as to face a drawing die 18 fixed in a holding case 17, as shown in Fig. 13A. As shown in detail in Figs. 14 to 16, the die 18 has a land portion 19 and a tapered portion 20 having cross sectional areas which become smaller as they approach the land portion 19. The inner face of the land portion 19 takes a substantially elliptical shape which is complementary to the outer peripheral surface of the fitting portion 14. The shape of the cross section of the inner face of the tapered portion 20 gradually changes from a circle to a substantial ellipse toward the land portion 19, and an intermediate portion of the tapered portion 20 takes a shape as shown in Fig. 16. After an end of the blank tube 16 has been disposed so as to face the die 18 at its large diameter side, a mandrel 21 is inserted into the die 18 from the opposite side to the blank tube 16 and then the front end portion 22 of the mandrel 21 is inserted into the blank tube 16, as shown in Fig. 13B. The shape of the front end portion 22 is similar to the shape of the inner face of the land portion 19, i.e., substantially elliptical. After the front end portion 22 of the mandrel 21 has been inserted into the blank tube 16, the blank tube 16 is pushed into the die 18. In synchronism with the pushing-in of the blank tube 16, the mandrel 21 is pulled out of the die 18. During this operation, the blank tube 16 is tightly held between the inner face of the land portion 19 and the outer peripheral surface of the mandrel 21 and plastically deformed so as to become substantially elliptical in cross section. After an elliptically cross-sectioned portion having an ample length has been formed by fully inserting the front end portion of the blank tube 16 into the die 18, the mandrel 21 is pulled out of the blank tube 16, as shown in Fig. 13D, and then the blank tube 16 is also pulled out of the die 18. The blank tube 16 having the required portion drawn so as to be substantially elliptical in cross section is transported to a station where annular grooves 7 are formed. However, the conventional method of manufacturing a hollow steering shaft in which the required portion of a blank tube 16 is drawn to form a substantially elliptical cross section is encountered with the following problems to be solved. (1) In order to manufacture a hollow steering shaft having a high quality, the dimensional accuracies, particularly the thickness accuracy must be controlled very severely. When the thickness is too large, use of the mandrel 21 as shown in Fig. 13C remarkably increases a shaping load required for pushing the blank tube 16 into the die 18. As a result, the buckling or the like adverse phenomenon occurs to the blank tube 12 due to the shaping load, hindering smooth shaping operation. When the thickness is too small, on the other hand, an intermediate part of each flat portion 13 of the elliptically cross-sectioned part of the blank tube 16 is bent inward, and/or the thickness of the connecting portions 23 between the flat portions 13 and the arcuate portions 12 becomes smaller than the required thickness. (2) An apparatus for inserting the mandrel and pulling the same out is required. This makes the structure of a shaping apparatus complicated and increases the apparatus cost. (3) Since the cross sections of the blank tube 16 are changed by holding the blank tube 16 between the land portion 19 of the die 18 and the mandrel 21 under a large force and by squeezing the blank tube 16 to reduce its thickness, the shaping load is very large. Thus, during shaping operation, a high surface pressure is applied to the land portion 19, and the land portion 19 and the outer peripheral surface of the blank tube 16 rub with each other, resulting in quick wear of the land portion 19. In consequence, the die 18 must be changed very frequently, creating a cause of a high manufacturing cost. (4) Since the shape of the cross sections is changed by reducing the thickness of the blank tube 16 as described above, the blank tube 16 is elongated during the drawing operation, and it is difficult to control the elongation accurately. Thus, a post-process is required for cutting the blank tubes 16 to the same length. This also creates a cause of a high manufacturing cost. When a hollow steering shaft is manufactured, the mandrel 21 which causes above-mentioned problems might be omitted. If, however, the mandrel 21 is omitted, intermediate parts of the flat portions of a blank tube are likely to be bent inward and/or the connecting portions between the flat portions and the arcuate portions of the blank tube are likely to have insufficient thickness, as is in the case where the thickness is too small. A hollow steering shaft might be manufactured by the use of a rotary swaging machine. In this case, a mandrel having an elliptical cross section is inserted into a blank tube having a circular cross section, and then the cross section of the blank tube is formed into a substantially elliptical shape by hitting the outer peripheral surface of the blank tube so that the inner diameter of the blank tube reduces. However, this manufacturing method requires a long operation time and a high apparatus cost, and noise is generated during the operation. Thus, this method is not practical. JP-A-61-219416, cited during substantive examination of this application, discloses continuous drawing of a blank pipe into a finished form having a non-circular cross-section. The blank is first passed through a first die where it is deformed, reducing its cross-sectional dimension along one transverse axis, and then through a second die where it is deformed, reducing its cross-sectional dimension along another transverse axis perpendicular to the first-mentioned transverse axis. Each die has a respective tapered drawing portion with a respective land portion adjacent thereto for squeezing the tube into a predetermined shape. Resultant opposite cross-sectional flat and/or arcuate portions of the finished pipe are respectively formed directly in one pass through a respective one of the dies. The method disclosed in JP-A-61-219416 requires a mandrel to be used for blank pipes having a wall thickness greater than 0.45mm. An object of the present invention is to overcome at least some of the above-mentioned disadvantages. Accordingly, the invention provides a method of manufacturing a hollow shaft including the steps of pushing part of a blank tube having a substantially circular cross section through a preliminary die and then through a finishing die and forming a tube having a pair of cross-sectionally arcuate portions and a pair of cross-sectionally flat portions, respective ones of said arcuate portions and said flat portions being arranged alternately around the circumference of said tube; each of said preliminary die and said finishing die comprising i) a respective tapered drawing portion having an inner periphery defining a respective first passageway having a cross-sectional area which becomes gradually smaller in a direction in which said tube is pushed into each said die and ii) a respective land portion adjacent an end of each said respective tapered drawing portion, each said land portion having an inner periphery defining a respective second passageway which has a smaller cross-sectional area than the smallest cross-sectional area of the adjacent respective first passageway, each said land portion squeezing said tube to form said tube into a predetermined shape; respective ones of a first pair of concave surface portions having a first radius of curvature and respective ones of a second pair of concave surface portions having a second radius of curvature being arranged circumferentially and alternately around the inner periphery of said preliminary die, said second radius of curvature being larger than said first radius of curvature; and respective ones of a third pair of concave surface portions and respective ones of a pair of flat surface portions being arranged circumferentially and alternately around the inner periphery of said finishing die. The method may further comprise the step of forming at least one circumferentially extending groove on the outer surface of a tube end portion which has been deformed by said dies. The invention also includes an apparatus for manufacturing a hollow shaft having a pair of cross-sectionally arcuate portions and a pair of cross-sectionally flat portions from a blank tube having a substantially circular cross section, respective ones of said arcuate portions and said flat portions being arranged alternately around the circumference of said shaft, said apparatus comprising: a preliminary die and a finishing die each having i) a respective tapered drawing portion having an inner periphery defining a respective first passageway having a cross-sectional area which becomes gradually smaller in a direction in which said tube is pushable into each said die and ii) a respective land portion adjacent an end of each said respective tapered drawing portion, each said land portion having an inner periphery defining a respective second passageway which has a smaller cross-sectional area than the smallest cross-sectional area of the adjacent respective first passageway, each said land portion being for squeezing said tube to form said tube into a predetermined shape; said land portion of said preliminary die comprising respective ones of a first pair of concave surface portions having a first radius of curvature and respective ones of a second pair of concave surface portions having a second radius of curvature arranged circumferentially and alternately around the inner periphery of said preliminary die, said second radius of curvature being larger than said first radius of curvature; and said land portion of said finishing die comprising respective ones of a third pair of concave surface portions and respective ones of a pair of flat surface portions arranged circumferentially and alternately around the inner periphery of said finishing die. In order that the invention may be well understood, an embodiment thereof will now be described, by way of example only, with reference to the accompanying drawings, in which: Figs. 1A to 1C are longitudinal cross sectional views of an apparatus used in various steps in a method of manufacturing a hollow shaft; Fig. 2 is a longitudinal cross-sectional view of a preliminary shaping die; Fig. 3 is a cross-sectional view along line III - III of Fig. 2; Fig. 4 is a cross-sectional view along line IV - IV of Fig. 2; Fig. 5 is a longitudinal cross-sectional view of a finishing die; Fig. 6 is a cross-sectional view along line VI - VI of Fig. 5; Fig. 7 is a cross-sectional view along line VII - VII of Fig. 5; Fig. 8 is a transverse cross-sectional view of a tube after having passed through the preliminary shaping die; Fig. 9 is a transverse cross-sectional view of the tube after having passed through the finishing die; Fig. 10 is a partially broken side view of a known steering apparatus in which a steering shaft is assembled; Fig. 11 is a side view of a hollow steering shaft; Fig. 12 is a cross-sectional view along line XII - XII of Fig. 11; Figs. 13A to 13E are longitudinal cross sectional views of the processes of a conventional method of manufacturing a hollow steering shaft; Fig. 14 is a longitudinal cross-sectional view of a shaping die used in the method of Figs. 13A to 13E; Fig. 15 is a cross-sectional view along line XV - XV of Fig. 14; Fig. 16 is a cross-sectional view along line XVI - XVI of Fig. 14; and Fig. 17 is a transverse cross-sectional view of the deformed tube after having been shaped. Figs. 1A to 7 show a manufacturing apparatus and a method of manufacturing a hollow steering shaft. A preliminary shaping die, or preliminary die, 25, a finishing shaping die, or finishing die, 26 and a correction die 27, all being fixed, are arranged concentrically in series in this order from the front side (the right side in Figs. 1A to 1C) to the rear side (the left side in Figs. 1A to 1C) with reference to the pushing direction of a blank tube 16 (in the left direction in Figs. 1A to 1C) in a fixed holding case 24. The preliminary shaping die 25 and the finishing shaping die 26 are disposed adjacent to each other, and the finishing shaping die 26 and the correction die 27 are connected by a spacer 28. The preliminary shaping die 25 has a shape as shown in Figs. 2 to 4 and comprises a tapered drawing portion, or drawing taper portion, 29 having an inner periphery defining a passageway whose cross-sectional area becomes gradually smaller toward the rear end of the die 25, a land portion 30 formed on the rear end of the drawing taper portion 29, and a tapered relief portion 31 having an inner periphery defining a passageway having a cross-sectional area which becomes larger away from the land portion 30. The land portion 30 squeezes the blank tube 16 and forms the same into a predetermined shape and has an inner peripheral surface having a shape as shown in Fig. 3. This peripheral surface of the land portion 30 comprises a first pair of (first) concave surface portions 32 having a smaller radius of curvature and a second pair of (second) concave surface portions 33 having a larger radius of curvature. Respective ones of the first and second pairs of concave surface portions 32 and 33 are arranged alternately around the circumference of the tube to form a substantially elliptical shape. The shape of the inner peripheral surface of the drawing taper portion 29 changes from a circle to an ellipse and becomes gradually smaller toward the land portion 30. For example, the shape of an intermediate part of the drawing taper portion 29 has concave portions 33a which are contiguous to the second concave surface portions 33 and have a radius of curvature smaller than that of the second concave surface portions 33. The finishing shaping die 26 has a shape as shown in Figs. 5 to 7. It comprises a drawing taper portion 34 having an inner periphery defining a passageway having a cross-sectional area which becomes gradually smaller toward the rear end of the die 26 a land portion 35 formed on the rear end of the tapered drawing portion, or drawing taper portion, 34, and a tapered relief portion 36 having an inner periphery defining a passageway having a cross-sectional area which becomes larger away from the land portion 35. The land portion 35 squeezes the blank tube 16 and forms it into a predetermined shape. As shown in Fig. 6, the land portion 35 comprises a pair of concave surface portions 37 and a pair of flat portions 38, which are formed in the finishing shaping die 26 and arranged circumferentially and alternately around the inner periphery of the die 26 to take a substantially elliptical shape. As shown in Fig. 7, the cross section of the drawing taper portion 34 has a substantially elliptical shape similar to the land portion 35. The inner peripheral surface of the drawing taper portion 34 is tapered in a conical manner so that the cross-sectional area of the passageway therein becomes smaller toward the land portion 35. The area of the opening at the front end of the finishing die 26 is larger than the cross-sectional area of the opening in the land portion 30 of the preliminary shaping die 25 so that the tube 16 which has passed through the land portion 30 is received in the taper portion 34. The correction die 27 tightly holds the outer peripheral surface of the tube 16 which has passed through the finishing shaping die 26 and corrects the shape of the outer peripheral surface of the tube 16. The correction die 27 has the same shape as the finishing shaping die 26. When a hollow steering shaft is manufactured by using the above-mentioned manufacturing apparatus, a blank tube 16 having a circular cross section is disposed in such a manner that the front end portion of the blank tube 16 faces the preliminary shaping die 25 as shown in Fig. 1A, and the blank tube 16 is pushed into the preliminary shaping die 25, as shown in Fig. 1B. As the blank tube 16 is pushed in, it passes through the preliminary shaping die 25 and then is inserted into the finishing shaping die 26. The tube 16 further passes through the correction die 27 and is formed into a hollow steering shaft having a substantially elliptical shape in cross section at an end portion thereof. The outer peripheral surface of the tube 16 which has passed through the preliminary shaping die 25 is formed into a shape in conformity with the shape of the inner peripheral surface of the land portion 30 of the preliminary shaping die 25, as shown in Fig. 8. After having passed through the land portion 30, the tube 16 is formed into a substantially elliptical shape defined by a first pair of opposite (first) convex surface portions 39 having a smaller radius of curvature and a second pair of opposite (second) convex surface portions 40 having a larger radius of curvature, which are arranged alternately around the circumference of the tube. Since the outer peripheral surface of the tube 16 is defined by convex surfaces provided over the whole circumference of the tube 16, the flat portions 41 are not bent inward as shown in Fig. 17, even if the mandrel 21 (Figs. 13A to 13E) is omitted. Thus, the shape of the outer peripheral surface of the blank tube 16 can coincide with the shape of the inner peripheral surface of the land portion 30 of the preliminary shaping die 25 with a high accuracy. The tube 16 which has passed through the preliminary shaping die 25 is guided as it is to the drawing taper portion 34 of the finishing shaping die 26 and pushed into the land portion 35 of the finishing shaping die 26. As a result, the second convex surface portions 40 formed on the tube 16 are deformed into flat portions 41, as shown in Fig. 9. Since the amount of deformation from the second convex surface portions 40 to the flat portions 41 is small, the shape of the flat portions 41 does not change by this deformation eg to bend inwards as flat portions 13 in Fig. 17. The tube 16, which has passed through the land portion 35 of the finishing shaping die 26 and is formed into a substantially elliptical shape in cross section as shown in Fig. 9, is fed to the correction die 27 so that the shape of the tube 16 in which high accuracies of straightness and the like are required is corrected. After each portion of the tube 16 has been formed into its required shape having required dimensions, the tube 16 is pulled out of the dies 25 to 27, as shown in Fig. 1C. Thereafter, the tube 16, part of which has been drawn into a substantially elliptical shape, is transported to another station in which the tube 16 is formed with circumferentially extending annular grooves 7 and the like. In this way, the tube 16 is formed into an upper shaft 6a of the steering shaft 1 (Figs. 11 and 12). Using the method of manufacturing a hollow steering shaft described above with reference to Figs. 1 to 9, a steering shaft having a good quality can be manufactured without making the dimensional accuracies high and without using a mandrel. Experiments made in order to confirm the technical advantages of the method will be explained below. The conditions of the experiments are as follows. Blank Pipes Material: STKM15A Outer Diameter: 21.7 mm Thickness: 2.6 mm Length: 380 mm Pipe to be Drawn: electric resistance welded blank tubes (blank tubes which are not yet drawn after having been electric-resistance welded) Shape to be Drawn: 21.5 mm x 16 mm (substantially elliptical) The blank pipes were drawn in three processes A, B and C. A. Conventional process The process in which a mandrel 21 was used as shown in Figs. 13A to 13E. B. Process for a Comparative Purpose ie for the purpose of comparison. The process shown in Figs. 13A to 13E but in which a mandrel 21 was not used. C. Process as described with reference to Figs. 1 to 9. The process in which a preliminary shaping die 25, a finishing die 26 and a correction die 27 were used as described above. The results of the experiments in the three processes are listed as follows: Processes Shaping Loads P (kgf) Bending of Flat Portions Having Elliptical Cross Section δ (mm) Contraction Loads Upon Collision W (Standard 150 - 300kg) Axial Elongations e (mm) (Variations Δe) Accuracies Required for Material (Variations of Thickness) A 3,400 -7,000 (Buckled) 0.01 - 0.05 210 - 280 9.0 - 14.9 (5.9) Not more than 0.05 mm B 2,600 0.09 - 0.12 100 - 500 4.0 - 4.3 (0.3) No limit C 2,600 Not more than 0.02 220 - 270 4.0 - 4.3 (0.3) No limit In the table, the shaping loads P are the forces required for pushing the blank tubes into the die. The smaller, the more preferable the shaping loads are. In the process A, an example of a steering shaft to which the shaping load of 7,000 kgf was applied was buckled and could not be drawn. The bending δ of flat portions 41 is the amount of the inward bending of intermediate parts of the flat portions, as shown in Fig. 17. When the flat portions 41 are bent inward in a steering shaft 1 formed by assembling an upper shaft 6a and a lower shaft 5 together, the loads required for shrinking the steering shaft 1 (contraction loads W at the time of collision) vary and become large. If the loads increase, the driver cannot always be protected well upon collision. Thus, the loads must be decreased and be stabilized. The axial elongation e is an amount of elongation of the overall length of a blank tube due to drawing operation. It is preferable that variation of the elongation be as small as possible in order to omit cutting operation of upper shafts 6a to a predetermined length. As apparent from the list showing the results of the above-mentioned experiments, the method described with reference to Figs. 1 to 9 has the following technical merits: (1) A hollow steering shaft can be manufactured without limiting the dimensional accuracies of the blank tube 16 including the thickness accuracy to strict values. (2) The structure of the shaping apparatus is simple and the apparatus cost is low because no mandrel is used. (3) The shaping loads are low because no mandrel is used and the shape of the cross section of the blank tube can be changed without reducing the thickness of the blank tube 16. Thus, the land portions of the shaping dies 25 to 27 are less worn and the shaping dies 25 to 27 need not be replaced so frequently. This results in a lower manufacturing cost. (4) The axial elongation of the overall length of a blank tube 16, due to drawing operation, not to mention the variations, becomes small, and the cutting operation of the blank tube 16 after the drawing operation can be omitted, then the manufacturing cost will be reduced. Upon working the method under the above-mentioned conditions, it was found that projection of the central part of each second concave surface portion 33 of the preliminary shaping die 25 at both ends of the die 25 by about 0.4 mm was preferable. Although not shown in the drawings, a preliminary shaping die 25 and a finishing shaping die 26 can be integrally formed by electric discharge machining or the like process. Using the method described with reference to Figs 1 to 9, a good quality steering shaft can be manufactured at low cost. According to the method of manufacturing a hollow steering shaft described herein with reference to Figures 1 to 9, part of the blank tube 16 is drawn so as to be formed into an elliptical shape in cross section, without using a mandrel and without limiting the wall thickness of the blank tube to a severe value. The shape of the outer peripheral surface of the blank tube 16 which has passed through the preliminary shaping die 25 is formed into a shape defined by the first pair of convex surface portions 39 having a smaller radius of curvature and the second pair of convex surface portions 40 having a larger radius of curvature in conformity with the shape of the inner peripheral surface of the land portion 30 of the preliminary shaping die 25, the first and second pairs of convex surface portions 39, 40 being arranged circumferentially and alternately. By preliminary shaping, the blank tube 16 is formed so as to have an outer peripheral surface consisting of convex surface portions over the whole circumference of the tube. The tube resists well against the forces applied in the direction in which the flat portions 13 are bent inward in the prior art. Thus, the shape of the outer peripheral surface of the tube 16 can coincide with the shape of the inner peripheral surface of the land portion 30 of the preliminary shaping die 25 at a high accuracy, even if a mandrel is not used. After having passed through the preliminary die 25, the tube is inserted into the finishing shaping die 26 and the second pair of convex surface portions 40 are formed into flat portions 41. Since the amount of deformation of the tube from the second pair of convex surface portions 40 to the flat portions 41 is small, the flat portions 41 do not lose their shape by this deformation.

Claims: A method of manufacturing a hollow shaft including the steps of pushing part of a blank tube (16) having a substantially circular cross section through a preliminary die (25) and then through a finishing die (26) and forming a tube having a pair of cross-sectionally arcuate portions (39) and a pair of cross-sectionally flat portions (41), respective ones of said arcuate portions (39) and said flat portions (41) being arranged alternately around the circumference of said tube; each of said preliminary die (25) and said finishing die (26) comprising i) a respective tapered drawing portion (29,34) having an inner periphery defining a respective first passageway having a cross-sectional area which becomes gradually smaller in a direction in which said tube is pushed into each said die (25, 26) and ii) a respective land portion (30,35) adjacent an end of each said respective tapered drawing portion (29), each said land portion (30, 35) having an inner periphery defining a respective second passageway which has a smaller cross-sectional area than the smallest cross-sectional area of the adjacent respective first passageway, each said land portion (30,35) squeezing said tube to form said tube into a predetermined shape; respective ones of a first pair of concave surface portions (32) having a first radius of curvature and respective ones of a second pair of concave surface portions (33) having a second radius of curvature being arranged circumferentially and alternately around the inner periphery of said preliminary die (25), said second radius of curvature being larger than said first radius of curvature; and respective ones of a third pair of concave surface portions (37) and respective ones of a pair of flat surface portions (38) being arranged circumferentially and alternately around the inner periphery of said finishing die (26). A method as claimed in claim 1, further comprising the step of forming at least one circumferentially extending groove (7) on the outer surface of a tube end portion which has been deformed by said dies (25, 26). An apparatus for manufacturing a hollow shaft having a pair of cross-sectionally arcuate portions (39) and a pair of cross-sectionally flat portions (41) from a blank tube (16) having a substantially circular cross section, respective ones of said arcuate portions (39) and said flat portions (41) being arranged alternately around the circumference of said shaft, said apparatus comprising: a preliminary die (25) and a finishing die (26) each having i) a respective tapered drawing portion (29, 34) having an inner periphery defining a respective first passageway having a cross-sectional area which becomes gradually smaller in a direction in which said tube is pushable into each said die (25, 26) and ii) a respective land portion (30, 35) adjacent an end of each said respective tapered drawing portion (29), each said land portion (30, 35) having an inner periphery defining a respective second passageway which has a smaller cross-sectional area than the smallest cross-sectional area of the adjacent respective first passageway, each said land portion (30, 35) being for squeezing said tube to form said tube into a predetermined shape; said land portion (30) of said preliminary die (25) comprising respective ones of a first pair of concave surface portions (32) having a first radius of curvature and respective ones of a second pair of concave surface portions (33) having a second radius of curvature arranged circumferentially and alternately around the inner periphery of said preliminary die (25), said second radius of curvature being larger than said first radius of curvature; and said land portion (35) of said finishing die (26) comprising respective ones of a third pair of concave surface portions (37) and respective ones of a pair of flat surface portions (38) arranged circumferentially and alternately around the inner periphery of said finishing die (26).

Inventors: OKUBO KIYOSHI, WATANABE YASUSHI, YOKOI KOICHI, OKUBO, KIYOSHI, WATANABE, YASUSHI, YOKOI, KOICHI
Applicants: NSK LTD
Application year: 1994
Main classification: B21C1
IPC codes: B21C3, B62D1, B21K1, B21C1, B21K21